1. Field of the Invention
This invention relates to a damping system for the suppression of vibrations created in machining processes and, more particularly, to a damping system for suppressing the free vibration and process instability associated with the use of various boring and cutting tools utilized in the machining arts.
2. Description of the Background Art
Many types of damping systems exist in the machining arts today for suppressing vibrations associated with metal cutting. Metal cutting tools and machining systems commonly exhibit detrimental vibrations during their operation. These vibrations can be classified into two types: (1) self excited vibration known as "chatter"; and (2) free vibration of the cutting tool due to a discontinuous cutting force being exerted on the cutting tool, such as in the case of an interrupted cut. Both types of vibration commonly lead to undesirable cutting performances such as poor workpiece surface finish and out of tolerance finished workpieces. Additionally, the cutting tools and the machines can become damaged due to the undesirable vibrations.
The overall machining system is a dynamic system of which multiple modes of vibration are involved in its operation. These multiple modes of vibration can be classified by their dynamic stiffness. The dynamic stiffness of a machining system is a measure of the machining system's resistance to deformation and the amount of damping in that particular mode of deformation. In the machining arts, the most dynamically flexible mode will dominate the performance of the machining system.
In situations susceptible to chatter, the dynamic stiffness of the most flexible mode determines the limit of stability for the overall system. In the chatter susceptible system, the limit of stability is the largest cut depth that can be obtained without the system becoming unstable and beginning to chatter. However, in the free vibration situation, the most dynamically flexible mode acts to contribute the majority of the uncontrolled displacement in the cutting tool position. This uncontrolled displacement is the dominant contributor to surface irregularities that occur in free vibration situations.
Certain machining system configurations typically exhibit a dominant mode of vibration. This dominant mode of vibration is significantly more flexible than the other modes of vibration associated with the particular system. This is commonly the case with long overhanging tools (tools with a high length to diameter ratio) such as boring bars, long end mills, modular tools and tool extensions. Further, these tools are often used in situations that are susceptible to vibration problems occurring due to interrupted boring, like what would be experienced when boring the cylinder of a two cycle internal combustion engine. In such cases, the overall performance of the machining system can be greatly enhanced by increasing the dynamic stiffness of the dominant mode of vibration.
There are several techniques for increasing the dynamic stiffness of a machining system as taught in the prior art. The aspect ratio of an over hanging tool may be reduced. That is, the length-to-diameter ratio may be reduced, but this option is not always possible due to the geometry of the cut required. The body of the tool may be made of a stiffer material such as tungsten carbide or some other heavy metal. In addition, a dynamic vibration absorber may be added to the system, with or without the application of the above mentioned remedies.
A complete discussion of dynamic vibration absorbers can be found in chapter 6 of the textbook entitled, "Shock And Vibration Handbook", authored by Cyril M. Harris and Charles E. Crede and published by McGraw-Hill Book Company (1961), the disclosure of which is hereby incorporated by reference herein. A dynamic vibration absorber can be tuned so as to vibrate at a desired frequency as is taught in U.S. Pat. Nos. 3,838,936 and 4,553,884. Further, a dynamic vibration absorber may be designed and tuned to reduce the minimum dynamic stiffness of which a machining system will operate at in order to avoid chatter, as is taught in U.S. Pat. No. 3,643,546.
However, the prior art damping systems have many inadequacies of which continue to limit the machining art. The current dynamic vibration absorbers (damping means) that are utilized today in the industry only provide a benefit in forced vibration applications undertaken at a specific excitation frequency and, therefore, do not effectively reduce either free vibration or chatter. It can be shown that the optimum tuning of a damping means is different for each of the individual vibration cases.
In order to optimally suppress vibration due to free vibration in interrupted cutting, the damping means should be tuned to minimize the absolute value of the "displacement versus force" transfer function for the machining system's most dynamically flexible mode of vibration. While in providing maximum resistance to chatter, the vibration damping means should be tuned to minimize the height of the negative peak of the "Real" part of the "displacement versus force" transfer function that is associated with the most dynamically flexible mode of vibration. The "Real" part refers to the displacement that is in-phase with the force.
Further, it must be realized that the machining system and the added damping means aggregate to form a strong overall dynamically dependent system. In other words, the addition of the damping means to the machining system acts to strongly affect the dynamic characteristics of the system. The affect is not simply due only to the addition of the damping means to the machining system but also due to the significant mass of the damping means itself relative to the effective modal mass of the machining system (the damping means, however, must be of a relatively large mass to have sufficient damping effect on the machining system). Thus, the machining system and the damping means may not be treated independently. In other words, the damping means cannot be tuned separately apart from the machining system and then added later. The simple addition of a damping means to a machining system will affect the frequency of the modes to be dampened and, therefore, will lead to a non-optimally tuned damping means.
Still further, any modification to the system will strongly affect the optimum tuning of the damping means, for example: the placing of a damped tool in a different machine; the changing of the length of the tool; the changing of the configuration of the damped tool in a modular tooling system; or the placing of the dynamic damping means in a different location in the machining system. In essence, the particular setup of the machining system with the added damping means acts to form a unique configuration of which is optimally tuned. The tuned damping means is, therefore, only optimized for that unique configuration.
Additionally, because the machining system and added damping means is highly dependent and the damping means will require periodic retuning, the damping means should be tuned while in position within the machining system. Since some personnel responsible for supervising the tooling or machining operation may not be familiar with structural dynamic tuning, there is a need for the damping means to be easily tunable while in position within the machining system with the use of only simple instructions and little or no prior knowledge of structural dynamic testing and tuning methods.
Therefore, it is an object of this invention to provide improvements of which overcome the aforementioned inadequacies of the prior art damping systems and provide an improvement which is a significant contribution to the advancement of the machining art.
Another object of this invention is to provide a tunable damping system having a tunable damping assembly and a tuner assembly of which automatically identifies the most dynamically flexible mode of vibration of a machining system having the added damping assembly configured and then procedurally directs the tuning so as to increase the dynamic stiffness of the most dynamically flexible mode.
Another object of this invention is to provide a tunable damping system of which the tuning and operation thereof requires little or no prior knowledge of structural dynamic testing and tuning methods.
Another object of this invention is to provide a tunable damping system whereby the actual tuning is electronically controlled and a user is instructed on how to adjust the damping assembly so as to be optimally tuned in either one or both of the following cases: (1) the case of minimum response due to free vibration, and (2) the case of maximum stability and resistance to chatter.
Another object of this invention is to provide a tunable damping system having a tuning capability and tuning procedure of which are insensitive to the location of the tunable damping assembly, the damping assembly's effective modal mass, and the effects caused by varying the configuration of the machining system. The damping assembly thereby being allowed to be placed at any convenient location within the tool, the tool extension, or the tool taper while still allowing sufficient movement of the tool in the vibration mode to enable the tunable damping assembly to operate effectively. The ability to vary the tool configuration, such as in modular tooling, without having to change the tuning procedure is also provided for.
Another object of this invention is to provide a tunable damping system that is conveniently usable in the production environment thereby allowing in situ tuning of the tunable damping assembly by the machine operator or other personnel.
Another object of this invention is to provide a tunable damping system that is to be utilized in or on fixed or rotating tools such as boring bars, end mills, modular tooling, tool extensions, spindles, tool holders or fixed machine structures.
These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be obtained by applying the disclosed invention in a different manner or by modifying the invention within the scope of the disclosure. Accordingly, other objects and a more comprehensive understanding of the invention may be obtained by referring to the summary of the invention, and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings.